Revert "Use xmlEncode for proper unicode handling"

This reverts commit c5cf87071c.

We can remove utf8_encode in stk_addons api so it will handle
utf8 characters in db correctly, as utf8_encode is only needed for
a string encoded in ISO-8859-1.

Fix #3670

src/online/http_request.hpp

@@ -143,7 +143,7 @@ namespace Online
         void addParameter(const std::string &name,
                           const irr::core::stringw &value)
         {
-            std::string s = StringUtils::xmlEncode(value);
+            std::string s = StringUtils::wideToUtf8(value);
 
             // Call the template to escape strings properly
             addParameter(name, s.c_str());